1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
2. Description of the Background Art
As a power semiconductor device as a switching element for driving a motor and the like, an IGBT (Insulated Gate Bipolar Transistor) is mainly used in a region where a regular voltage is 300V or more.
As such a power semiconductor device, a constitution, in which two types of trench inside IGBT cells are provided for both a gate-trench and a non-gate trench filled with a conducting material directly connected to an emitter electrode to have the same potential of emitter region not a gate, has been conventionally proposed (refer to Japanese Patent Laying-Open No. 2002-353456, and Pamphlet of International Publication No. WO 02/058160).
Especially, according to the Japanese Patent Laying-Open No. 2002-353456, the gate electrode of the IGBT and the filling layer having the emitter potential are formed in the same step.
According to the manufacturing process, a trench for the gate electrode and a trench for the filling layer are first formed in a substrate, and a first insulation film is formed so as to cover an inner wall of each of the trenches. Then, a conductive layer is formed on a whole surface of the substrate so as to fill the trenches, and the conductive layer is fully etched away. Thus, the conductive layer remains only in each of the trenches, whereby the gate electrode and the filling layer are formed.
Then, a second insulation film is formed on a whole surface of the substrate so as to cover the gate electrode and the filling layer, and the second insulation film is selectively etched away. Thus, a contact hole exposing the periphery of the filling layer is formed on the second insulation film, and the second insulation film on the gate electrode remains. Thereafter, when an emitter electrode is formed on a whole surface, the emitter electrode is electrically connected to the filling layer through the contact hole, and electrically insulated from the gate electrode by the second insulation film.
Thus, the gate electrode of the IGBT and the filling layer having the emitter potential are formed in the same step.
However, according to the constitution and manufacturing method disclosed in the Japanese Patent Laying-Open No. 2002-353456, a fine gap is generated between the filling layer and the trench inner wall, and an aluminum spike is generated at that part, which lowers the reliability. This will be described hereinafter.
According to the manufacturing method disclosed in the Japanese Patent Laying-Open No. 2002-353456, several tens % of the thickness of the second insulation film is processed by overetching in general in the etching process for forming the contact hole. This overetching is performed in view of the variation in thickness of the second insulation film on a wafer surface and between wafer surfaces and variation in etching speed of an etching equipment.
A predetermined amount of the first insulation film formed between the filling layer and the inner wall surface of the trench is etched away by this overetching. Thus, an extremely small gap as much as the thickness of a gate oxide film is generated between the filling layer and the inner wall surface of the trench.
In addition, before a high melting point metal to form silicide by contact with silicon is formed by sputtering or the like, the surface is etched with fluorinated acid (HF) in order to remove a natural oxide film of the exposed part of silicon in addition to general cleaning of the contact hole part using acid or alkaline fluid. Also at this time of etching, a predetermined amount of the first insulation film formed between the filling layer and the inner wall surface of the trench is etched away. Thus, the first insulation film between the filling layer and the inner wall surface of the trench is further deeply etched away.
The gap generated as described above is as fine as a processing dimension of the most advanced LSI (Large Scale Integrated Circuit) and has a sectional structure that can be regarded as a double contact hole generated in the contact hole. Therefore, even when a sputtering apparatus used for the most advanced LSI is used, it is extremely difficult to fill this gap with a metal film such as titanium (Ti) film as a barrier layer. Even when a metal film is put on the gap, it is inevitable that the film is thinned and a pinhole is generated.
As a result, aluminum as an emitter electrode material and silicon as a substrate material are directly reacted through the metal film having a low barrier property by a heat treatment in the following step or electro migration reaction generated when a current is applied for a normal element operation. Thus, silicon is diffused in aluminum, and at the same time, aluminum eats away in the silicon as a spike (that is, an aluminum spike is generated), so that the electric characteristics are considerably damaged and long-term reliability cannot be maintained.